Determination of hydrocarbon concentration in soil samples



March z, 1948. E. E. nor-1ER `,2,437,045

DETERMINATION OF HYDROCARBON CONCENTRATION IN4S0IL SAMPLES I Filed April 24, 1944 Joure di?" ffl/2f all Patented Mar. 2, 1948 UNITED STA'lE-Sl PATENT OFFICE DETERMINATION OF HYDROCARBON'CON- CENTRATION IN SOIL SAMPLES Edwin.r E. Roper, Tulsa, Okla., assignor to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation of Delaware Application April- 24, 1944, Serial No. 532,427

the present invention relates to a method of analyzing samples of soil or rock to evaluate the concentration of the upwardly diffusing petroliferou-s material arising from said subterranean deposits. The presence of said material in the soil sample is an indication of underlying relatively deep-seated petroliferous deposits. When using the method of the present invention, the concentration of such material found at; or near the surface of the earth accurately reflects the lateral extent of thev deep-seated' petroliferous deposit.

Itis well known to analyze samples of the soil by physico-chemical decomposition methods to determine the presence and quantity of hydrocarbonsj in the soil, said hydrocarbons originating primarily from an underlying petroliferous deposit. However, the quantity of hydrocarbon found in the soil sample can not, in general, be directly utilized as a geochemical prospective index. The reason for thisv is to be found in the Widely varying sorptive capacities ofv different soil or rock samples. This is explainedl in detail in U. S. Patent 2,165,440, which describes a method for ascertaining the sorptive capacity of each individual soil sample whereby the quantity of hydrocarbon found in each sample may be corrected to compensate for the varying sorptive capacities. This patent teaches that the' sorptive capacity of each soil sample must be determined in order to correct the corresponding hydrocarbon concentration if a reliable survey is to be obtained. This is necessary so that all the soil samples of the geochemical survey will be on a strictly comparable'basis. The corrected values of hydrocarbon concentration will then truly reflect the position of the subterranean petroliferous deposit, since said corrected values are directly related` to the partial pressure of the upwardly-migrating hydrocarbons. According to the procedure described in the patent, it should be noted that the properties of the soil cancel out, and in effect, the soil itself serves merely as an intermediate or as a transfer medium.

The prior processes of analysis of soil samples by physico-chemical decomposition, however, involvenumerous disadvantages. According to said prior-known processes, the hydrocarbons sorbed in thesoil sample are removed either by the application of a vacuum, as described. in my U. S. application S. N. 378,785, by the application of heat, as described in U. S. Patent 2,212,681, issued to T. H. Dunn, by means of a stream of sweeping gas, as described in my U. S. application S. N. 452,842, by the addition of a suitable chemical reagent, or bythe various possible combinations 2 l of these fou-r methods. The recovered hydrocarbons are subsequently determined by analysis employing means well known to those skilled in the' art. One of the principal objections to such methods is that there is no certainty that all of the sorbedA hydrocarbons are completely removed, for in most instances it is exceedingly diiiicult to completely remove all of the sorbed hydrocarbons even when excessive temperatures, prolonged evacuation, or numerous consecutive extractions are employed. `If too high temperatures are utilized in an attempt' at more thorough removal, pyrolysis of the hydrocarbons takes place, giving rise to erroneous results. In addition to analyzing each soil sample for sorbed hydrocarbons it is generally necessary to determine the sorptive capacity of each soil sample thus prolonging the time andI elfort required.

It is therefore an object of the present invention to provide a process in which a true and reliable prospecting index may be determined by performing butone type of measurement on the' soil sample.. An` additional object is to provide a proces-s in which variationsv due to the sorptive capacities of the soil are inherently and automatically overcome. A further object of the present invention is to provide a process which may be carried out at normal temperatures so that changes in the nature of the hydrocarbons due to pyrolysis can not take place. Still another object of the present invention is to providek a process which may be carried out more easily and sim-ply than previously-known processes, and which will give results that are more accurate an reliable than heretofore.

Fundamentally, the process of the present invention is a synthetic method of analysis, contrasted to those previously-known, which are all decomposition methods. A portion of a soil sample after collection is immersed in an artificial atmosphere containing a known quantity of hydr'ocarbons and after a suitable period of time the amount of hydrocarbon in the articial atmosphere ismeasured by some means. The gain or loss of hydrocarbon in the artificial atmosphere is then a measure of the disparity of the partial pressure of the hydrocarbon in the artificial atmosphere and that of the equilibrium pressure of the hydrocarbon sorbed on the soil sample. By repeating the measurement with a second fresh portion of the s-oil sample and an articial atmosphere containing a different known partial pressure of hydrocarbon, the true partial pressure of the sorbed hydrocarbon in the original soil sample can be computed. It can be seen that vas close an approach to the true partialv pressure can be established as is desired by repeating such measurements many times. However, it has been found that this is not generally necessary and that from twofto four measurements are all that are required. Alternatively, a method may be employed in which concurrent determinations are made rather thanV consecutive determinations. This method has been termed an equilibration method, whereby a successive approximation approach to the true gr'eater than the gas equilibrium concentration of sorbedrhydrocarbons on the soil sample. This will be true, since the soil sorbed some of the equilibrium conditions of the sorbed hydrocarbon Y gas in situ is obtained.

It Will be appreciated from this description of the general method of operation of the present invention that the properties of the soil cancel out, and in effect, the soil merely serves as an intermediate or as a transfer medium. This is somewhat analogous to the process described in U. S. Patent 2,165,440 in principle but the method of the present invention accomplishes this and an additional step in one simple, direct measurement. In addition, the data secured in the application of the present method permits the sorptive capacity of the soil to be computed if this proves necessary and helpful.

The following description is a preferred technique which will permit one skilled in the art to reproduce the method in accordance with the concept of the present invention. However, nothing herein is to be construed as limiting the scope of the present invention to the preferred technique, since the same fundamental oper-V ations may be accomplished by other means.

The drawing illustrates an example of an apparatus which can be used for carrying out the process described hereinafter. The artificial atmospheres may be prepared by adding any known amounts of hydrocarbons to inert gas such as nitrogen, helium, argon, hydrogen, or air, the rst named being preferred. It is advantageous to initially analyze the atmospheres by the same method as is subsequently used for determining the hydrocarbon concentration of the atmosphere after equilibrium with the soil sample has been obtained. Theoretically, in order to avoid loss of water in the soil sample and consequently change the sorptive capacity of the soil, the partial pressure of water vapor in the articial atmosphere should be equal to the equilibrium partial pressure of the water over the soil sample. Actually, it has been found that after the articial atmosphere is saturated with water vapor no erroneous results will occur. The artiiicial atmosphere can be easily saturated with water vapor bypassing the inert gas through water. According to the preferred method of the present invention three artificial atmospheres are used, containing respectively, 30, 60, and 120 P, P. M. (parts per million) of hydrocarbon.

After a soil sample has been collected and divided into separate portions, one of the portions is hermitically sealed in a container and subjected to a vacuum for a short interval of time in order to deaerate the soil sample.

Following the deaeration a known amount of one of the articial atmospheres is introduced `into the container. The soil sample is permitted to remain severalhours to several days. It has been found that Ya period of 18 hours is highly satisfactory for the purposes of the present invention. At the end of this period of time, the hydrocarbons sorbed on the soil sample are in substantial equilibrium with the hydrocarbons in the articial atmosphere. The concentration of hydrocarbons in the atmosphere is then determined by any well-known method and this new concentration is compared with the original concentration of hydrocarbons in the articial atmosphere. Whenever there is a decrease in the hydrocarbons originally contained in the articial atmosphere. The same procedure is followed with another portion of the same soil sample, the concentration of hydrocarbons in the articial atmosphere, however, being reduced or increased. If the concentration of hydrocarbons is reduced, for example, and it is found, after substantial equilibrium has been obtained, that the concentration of hydrocarbons in the artiflcial atmosphere is greater than the original concentration, then it will be known that the gas equilibrium concentration of the soil sample was greater than the original concentration of hydrocarbons in the artificial atmosphere. By interpolating the data so obtained it is possible to compute that concentration of hydrocarbons in an artificial atmosphere, which would undergo no change during the period of time for estabiishing substantial equilibrium and which therefore corresponds to the true equilibrium condition of the sorbed hydrocarbon gas in situ. In other words, the original sorption equilibrium pressure of the gaseous hydrocarbons contiguous with the phase sorbed on the soil When it was in the earth may be ascertained.

While it is also desirable to use -a hydrocarbon atmosphereY containing hydrocarbons which are of substantially the same type as those contained in the sample of soil, this also is not essential. Such a refinement, however, can be carried out bytesting a portion of the soil sample qualitatively before carrying out the method of the present invention. Highly satisfactory and accurate results, however, can be obtained by using a. hydrocarbon atmosphere containing known concentrations of pure alkanes such as propane, butane, pentane, hexane, heptane, or suitable mixtures of the same. Obviously the temperature of the equilibrium step must be such that the hydrocarbons in the atmosphere are in the gaseous state. For this reason it may be more vdesirable to use hydrocarbons which are normally gaseous, or have appreciable vapor pressure, at normal temperatures, such as propane, butane or pentane.

The data presented in Table 1 show how the concentration, in parts per million, of the hydrocarbons sorbed on the soil sample may be computed. On occasion the desired information may be indicated as the partial pressures of the hydrocarbons in the hydrocarbon atmosphere rather than the concentration in parts per million. Such partial pressures may be obtained by multiplying the concentration C by a factor, 0.76,

according to known physical laws.

Table I Concentration 0f 01H12 in B. P. M. (by volume) p, Partial ressure 0l Elpt. C f f t l c-c/IO .Illu in gas o. o ar 1 cia p ase a rtifgli agnosphelrio equllibrluml a er equ ,i mospliere bratmn With reference tcl the drawing, which illustrates: one form of apparatus for carrying out the above-described process, a portion of a. soil sample is weighed (Ms grams). into container II which is then connected to the apparatus at I2. Now, in order to. determine the free gas! space in container Il, vr, stopcocks I3, I4, I5, I6, II, I3, I9, and ZI are closed. Stopcock I9 isV then slowly opened so as tov establish communication with a vacuum pumpor the like (not shown) until the pressure indicated on manometer 23 decreases from the prevailing barometric pressure to about l mm. This probably takes about two minutes and the pressure is maintained at about l mm. for an additional two minutes to completely evacuate the system. Stopcocks I9 and 22 are then closed. Helium is then introduced into the system by opening stopcock I4 until a high pressure of about 760mm. is indicated by manometer 23. The volume Va of the space dened by stopcock 22, calibration mark 24 on manometer 23, stopcocks I4, I9, 2I and I6 is known from a previous calibration. After the helium pressure has been built up, stopcock I4 is closed and the pressure, P2, is measured on manometer 23. The ambient temperature t2 is also measured and' recorded. Stopcock 22 isv .then opened andi the helium is permitted to expand into container II. This pressure, P3, and the ambient, t3, is-deter mined and recorded. From this data the volume of vs may be calculated from the equation:

Following the determination of the volume er, stopcock I9 is opened and the helium is withdrawn from the system by a vacuum pump or the like until the pressure drops to about 1 mm., as indicated on manometer 23. Stopcocks I 9 and 22 are then closed. Stopcock I is then opened to permit water to flow into flask containing an artificial atmosphere so that a pressure eXceed ing 760 mm. will be created therein. Stopcock I5 is then closed, and stopcocks I6 and 2I are opened so that a su-itable pressure of about 700 mm. is built up in gas-sampler flask 26. Flask 26 is then detached at 21, after closing stopcocks 2| and 20. The contents of flask. 26 are then analyzed by any suitable method in order to determine the concentration c4, of hydrocarbons in the artificial atmosphere contained in flask 25. This concentration is determined in P. P. M. (parts per million).

Stopcocks 22 andIB are opened so as to build up a suitable pressure in the container I I. The pressure, P4, is measured on manometer 23. The articial atmosphere is permitted to remain in contact with the soil sample in container II for a suitable period of time, for example, 18 hours, so that equilibrium will be established. During this time gas-sampler flask 26 is reconnected to connector 2'I and is evacuated thru stopcock I8, which is then closed. After the period for establishing equilibrium, stopcock 2I is opened and the gas in container II is permitted to expand into gas-samplerV flask 26. If desirable or necessary, stopcock I3 may be opened so that the gases may be positively expelled from container II. After the expansion of the gases,y stopcock 2l is closed and the pressure, P5, is measured on manometer 23. Stopcock 20 is then closed, ask 26 removed from connector 21, and the contents of flask 26 are analyzed for hydrocarbon concentration, C4, as before. If desired, the hydrocarbon concentration can be determined by the 6i' method described lin my lll. S.. application .S.-N. 452,842.

The procedure described above may be repeated again or several times, using fresh portions of the same soil sample and articial atmospheres having different. hydrocarbon lconcentrations c5, ce, etc., thus obtaining the. value of C5 as explained above. For calculating the true, original, in situ, concentration of gaseous hydrocarbons in the soil sample, Ce, the follow-ingV equation may be solved:

Having now described my invention and a` manner of carrying out same, what I claim is:

1. A method for determining an index of the concentration of hydrocarbons-sorbed on a soil sample, comprising subjecting each of a plurality of cle-aerated, substantially similar portions of said soil sample to an articial atmosphere, each of said atmospheres initially containing a different known concentration of hydrocarbons, maintaining each of said atmospheres in contact with the corresponding portion of said soil sample for a substantial period of time suiiicient to establish substantial equilibrium between the partial pressures of the hydrocarbons in the artificial atmosphere and the hydrocarbons sorbed on the soil sample, then determining the concentration of hydrocarbons in each of said atmospheres, Whereby said index may be determined.

2. A method for determining an index of the concentration of hydrocarbons sorbed on a soil sample, comprising removing air from a plurality of substantially similar portions of said soil sample, subjecting each portion to an artificial atmosphere for a substantial period of time sulcient to esta'blish substantial equilibrium between the partial pressures of the hydrocarbons in the artificial atmosphere and hydrocarbons sorbed on the soil sample, each of said atmospheres initially containing a diiierent known concentration of hydrocarbons, and determining the change in concentration of hydrocarbons in each atmosphere after said substantial period of time, whereby an index may be determined. n

3. A method for determining an index of the concentration of hydrocarbons sorbed on a soil sample, comprising removing air from a first portion of a soil sample, subjecting said iirst portion to a rst articial atmosphere for a substantial period of time suilicient to establish substantial equilibrium between the partial pressures of the hydrocarbons in the articial atmosphere and the hydrocarbons sorbed on the soil sample, said first atmosphere initially containing a known concentration of hydrocarbons, removing air from at least one other portion of said soil sample, subjecting each of said at least one other portion to an atmosphere for a period of time equal to said iirst-mentioned period of time, each of said atmospheres initially containing a concentration of hydrocarbons different from each other and from said rst atmosphere, and determining the concentration of hydrocarbons in each atmosphere, whereby an index may be determined.

4. A method for determining an index of the concentration of hydrocarbons sorbed on a soil sample comprising removing air from a rst portion of said soil sample, subjecting said first portion to a first artificial atmosphere for a, substantial period of time suiicient to establish substantial equilibrium between the partial pressures of the hydrocarbons in the artificial atmosphere and the hydrocarbons sorbed on the soilsample, said rst atmosphere initially containing a known concentration of hydrocarbons, removing air from at least two other portions of said sample, subjecting each ofsaid portions to an atmosphere for a period of time equal to said rst-mentioned period of time, one ofY said atmospheres containing an initial concentration of hydrocarbons higher than said rst atmosphere and at least one of the other of said atmospheres initially containing a concentration of hydrocarbons lower than said rst atmosphere, and determining the concentration of hydrocarbons in each atmosphere, whereby an index may be determined.

5. A method for determining an index of thev concentration of hydrocarbons sorbed on a soil sample, comprising dividing said soil sample into at least two portions, removing air from said portions, subjecting each of said portions to articial atmospheres for equal substantial periods of time suiicient to establish substantial equilibrium between the partial pressures of the hydrocarbons in the artificial atmosphere and hydrocarbons sorbed on the soil sample, each of said atmospheres initially containing different known concentrations of hydrocarbons, the concentra- -tion of hydrocarbons in one of said atmospheres being higher than the expected concentration of hydrocarbons sorbed on said soil sample and the concentration of hydrocarbons in at least one oi said other atmospheres being lower than the expected concentration of hydrocarbons sorbed'on said sample and determining the concentration .of hydrocarbons in each atmosphere after said substantial period of time, whereby an index may be determined.

EDWIN E. ROPER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITEDSTATES PATENTS Number Name Date 2,321,293 Hassler June 8, 1943 2,349,472 Taggart i May 23, 1944' 

